[media] media: Media Controller fix to not let stream_count go negative

Change media_entity_pipeline_stop() to not decrement
stream_count of an inactive media pipeline. Doing so,
results in preventing starting the pipeline.

Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>